Dampers used especially in the suspension of motor vehicles and for similar purposes have damping characteristics which are a compromise between the various objectives sought after, particularly comfort, stability, adhesion, etc. This compromise is the result of subjective assessments, because ideal damping varies depending on whether the same vehicle is used for relaxed motoring or sports driving. In the present state of the art, the compromise is arrived at empirically.
Moreover, the present tendency to produce lighter and ligher motor vehicles with suspensions of variable flexibility means that a single damping setting no longer meets the requirements of all the versions of one and the same vehicle model or all the motoring conditions and the drivers' wishes.
A hydraulic shock absorber of telescopic construction used especially in a motor-vehicle suspension, performs two functions: it brakes the movements of the suspended mass, in this particular case the body of the vehicle, and it controls the rebound of the wheels, these two phenomena giving rise to the main effects of resonance. The present invention relates mainly to the braking of the movements of the suspended mass, that is to say the body where a vehicle is concerned, this braking being ensured as a result of the low-speed damping rate of the suspension (up to approximately 0.2 m/sec., as measured vertically relative to the vehicle wheel). This part of the damping characteristic is mainly ensured conventionally, in a telescopic hydraulic shock absorber, by one or more permanent passages associated with a flap system. To modify the damping of such a hydraulic shock absorber, it is therefore sufficient to vary the cross-section of these permanent passages.
Telescopic hydraulic shock absorbers having an adjustment system capable of controlling the damping rate remotely are already known. In these shock absorbers of a known type, a means of shutting off the permanent passage in the flap system can be moved by means of a mechanical connection which connects the interior of the shock absorber to an actuation means located outside the shock absorber. Such an actuation means conventionally comprises a screw wheel or an electromagnetic device having a solenoid or alternatively an electric motor. Such devices can be used only under certain specific conditions. In particular, it is appropriate if a suitable space can be used outside the shock absorber to accommodate such a solenoid or such an electric motor. It is also expedient if a mechanical connection is possible between the shut-off means and the external actuation means.